1. Field of the Invention
The invention relates to a display device (referred below to an FED (Field Emission Display)), which makes use of electron source elements (electron emitting elements). Also, the invention relates to a method of driving the FED. Further, the invention relates to an electronic equipment making use of the FED.
2. Description of the Related Art
An explanation will be given to an FED (Field Emission Display) making use of an electron source element. Here, an element emitting electrons owning to the electric field effect is referred to as an electron source element.
Electron source elements arranged on respective pixels of the FED emit electrons from electrodes due to the electric field effect. Electrons thus emitted are accelerated to be incident upon a fluorescent body. The fluorescent body in a region, upon which electrons are incident, emits light. A quantity of electrons emitted from the electron source elements on the respective pixels is controlled by a video signal input into the FED. The more electrons emitted, the higher emission luminance of the fluorescent body in the case where these electrons are incident upon the fluorescent body. Thus the FED represents gradation.
Electron source elements have various configurations. There are typically given an FE (Field Emission) type element for causing electrons to be emitted from a tip end of a convex electrode where an intense electric field is locally generated, a surface conduction type element for causing generation of electrons through flowing of an electric current in parallel to a thin film surface broken locally, an MIM (Metal-Insulator-Metal) type element composed of a first electrode, a second electrode and an insulating film interposed between the first electrode and the second electrode, and for emitting electrons upon application of voltage between the first electrode and the second electrode.
Here, what is regarded as important in electron source elements used in FEDs is whether elements can be made minute, or whether elements having a uniform performance can be fabricated, or whether elements can be driven with low voltage. Hereupon, MIM type electron source elements meeting these qualifications have been developed.
FIG. 6 shows an example of an MIM type electron source element. Its structure is described in SID 01 Digest page 193-195 “Novel Device Structure of MIM Cathode Array for Field Emission Displays”.
In FIG. 6, formed on a substrate 20 with an insulating surface are a lower electrode 21, an upper electrode 23, and an insulating film 22 interposed between the lower electrode 21 and the upper electrode 23. Also, the reference numeral 24 denotes a protective insulating layer, 25a a contact electrode, 25b an upper electrode bus line, and 26 a protective electrode. In addition, a region where the upper electrode 23 overlaps an opening of the protective insulating layer 24 is referred to as an electron emission region and denoted by the reference numeral 27 in the figure.
Application of voltage between the upper electrode 23 and the lower electrode 21 causes injection of a hot carrier into the insulating film 22. That hot carrier of the hot carrier thus injected, which has a greater energy than a work function of a material of the upper electrode 23, passes through the upper electrode 23 to be emitted into the vacuum.
An MIM type electron source element having the structure shown in FIG. 6 emits electrons when voltage of around 10 V is applied between the upper electrode 23 and the lower electrode 21. In electron source elements, voltage applied between an upper electrode and a lower electrode when electrons are emitted is referred to as a drive voltage of an electron source element. An upper electrode of electron source elements is set to be high in electric potential as compared with a lower electrode thereof. In this manner, electrons are emitted from the upper electrode.
FIG. 7 shows an example of a display (FED) making use of the electron source element shown in FIG. 6. In addition, the same parts as those in FIG. 6 are denoted by the same reference numerals.
The FED shown in FIG. 7 has on the first substrate 20 with an insulating surface x (natural number) signal lines S1 to Sx arranged in a row direction, and y (natural number) scanning lines G1 to Gy arranged in a column direction. Electron source elements are arranged on respective points of intersection of the x signal lines S1 to Sx and the y scanning lines G1 to Gy. One electron source element, and that part of the signal lines and the scanning lines, to which the electron source element is connected, constitute one pixel. In FIG. 7, the reference numeral 300 denotes one pixel. The lower electrode 21 of the electron source element is connected to one of the y scanning lines G1 to Gy, and the upper electrode 23 is connected to one of the x signal lines S1 to Sx.
In addition, the lower electrode 21 may be connected to one of the x signal lines S1 to Sx and the upper electrode 23 may be connected to one of the y scanning lines G1 to Gy.
A second substrate 19 is provided to face that surface of the first substrate 20, on which the electron source element is provided. The second substrate 19 is light-transmissive. Arranged on the second substrate 19 is a fluorescent body 18 opposite to the electron source element. A black matrix 15 is arranged around the fluorescent body 18. In addition, the fluorescent body 18 is formed on a surface thereof with a metal-backed layer 17. Vacuum is kept between the first substrate and the second substrate.
A signal input into the scanning lines and a signal input into the signal lines cause emission of electrons from the upper electrode 23 in the electron source element of the pixel, in which voltage is applied between the upper electrode 23 and the lower electrode 21. Electrons thus emitted are accelerated in the vacuum 16 by voltage applied between the metal-backed layer 17 and the upper electrode. Electrons thus accelerated are incident upon the fluorescent body 18 provided on the second substrate 19 through the metal-backed layer 17. Thus the fluorescent body 18 in a region where electrons are incident emits light.
Here, a signal input into, for example, the scanning lines are kept constant in amplitude, and a signal input into the signal lines is varied in amplitude. A quantity of electrons emitted from the electron source element 28 is increased in accordance with voltage applied between the upper electrode 23 and the lower electrode 21. The more electrons emitted, the higher emission luminance can be represented in the case where these electrons are accelerated to be incident upon the fluorescent body 18 on the second substrate 19.
FIG. 8 shows a timing chart in the case where the display having the structure shown in FIG. 7 is driven. In the timing chart, one frame period (F) is a period, in which one picture image is displayed.
First, a scanning line G1 is selected. Here, other scanning lines G2 to Gy are put in a state, in which they are not selected. In addition, selection of a scanning line in FIGS. 7 and 8 means putting a scanning line connected to one of electrodes of an electron source element in a certain electric potential so that a quantity of electrons emitted from the electron source element is varied in accordance with an electric potential input into a signal line connected to the other of the electrodes of the electron source element.
For example, suppose that an electric potential of −8 V is input into a scanning line as selected in the case where a scanning line is connected to the lower electrode 21 of the electron source element and a signal line is connected to the upper electrode 23. On the other hand, suppose that an electric potential of 8 V is input into scanning lines as not selected. Also, suppose that an electric potential of −8 to 8 V is input into a signal line. Here, suppose that the upper electrode 23 of the electron source element emits electrons when the upper electrode 23 of the electron source element is higher about 10 V in electric potential than the lower electrode 21. At this time, the electron source element emits electrons when a signal electric potential of 5V from the signal line is input into the upper electrode 23 of the electron source element, of which the lower electrode 21 is connected to a scanning line in a selected state. Meanwhile, even when a signal electric potential of 5 V is input into the upper electrode 23 of the electron source element, of which the lower electrode 21 is connected to a scanning line in a non-selected state, the upper electrode 23 of the electron source element is lower in electric potential than the lower electrode 21 and so electrons are not emitted.
A period, in which the scanning line G1 is selected, is referred to as a first line period (L1). At this time, signals are successively input into the signal lines S1 to Sx. The electron source element emits electrons from the upper electrode 23 in accordance with signals as input. Thus emitted electrons cause the fluorescent body 18 provided on the opposed substrate 19 (second substrate) to emit light. In this manner, pixels in the first column emit light in accordance with signals as input. Subsequently, a scanning line G2 is selected. Here, G1, G3 to Gy are in a non-selected state. A period, in which the scanning line G2 is selected, is referred to as a second line period (L2). At this time, signals are successively input into the signal lines S1 to Sx. The electron source element 28 emits electrons from the upper electrode 23 in accordance with signals as input. Thus emitted electrons cause the fluorescent body 18 provided on the opposed substrate 19 (second substrate) to emit light. In this manner, pixels in the second column emit light in accordance with signals as input. The same action is repeated for all the gate signal lines, and so the one frame period is terminated. Thus the FED represents a picture image.
Since the above drive method is a passive type one, however, signals are directly input into electrodes of electron source elements of those pixels, on which display device should not be made. Therefore, there is involved a problem that power consumption is increased.
Hereupon, Japanese Patent Laid-Open No. 84927/2001 proposes an FED, in which a thin-film transistor (referred below to as a TFT) is arranged on each pixel. The constitution of this FED is shown in FIG. 9. FIG. 9 schematically shows electron source elements 902. The reference numeral 903 denotes lower electrodes, and 904 upper electrodes.
In FIG. 9, one of a source region and a drain region of a TFT 901 (referred below to as a pixel TFT) arranged every pixel is connected to one of x (natural number) signal lines S1 to Sx, and the other of the regions the lower electrode 903 of the electron source element 902. Also, a gate electrode of the pixel TFT 901 is connected to one of y (natural number) scanning lines G1 to Gy. The upper electrode 904 of the electron source element 902 is kept at a certain electric potential Vcom.
A selection signal is input into the scanning lines G1 to Gy. A pixel TFT 901 connected to a scanning line, into which a selection signal is input, is made ON. A signal input into a signal line is input into the lower electrode 903 of the electron source element 902 through the pixel TFT 901 having been made ON.
The electron source element 902 emits electrons due to a difference between an electric potential of the signal input into the lower electrode 903 and an electric potential of the upper electrode 904. Thus emitted electrons cause the fluorescent body to emit light, and so the pixel emits light. In addition, when the electron source element 902 emits electrons from the upper electrode 904, the upper electrode 904 is kept higher in electric potential than the lower electrode 903.
Power (reactive power) consumed in those pixels, in which display device should not be made (signals are not input into both scanning lines and signal lines), can be significantly reduced in a display device constructed such that the pixel TFT 901 is arranged in each pixel and a signal from a signal line is input into the lower electrode 903 of the electron source element 902 only in a pixel, in which the pixel TFT 901 is made ON.
An MIM type electron source element emits electrons when voltage is applied between an upper electrode and a lower electrode. Therefore, with pixels of a display device constructed in the manner described in Japanese Patent Laid-Open No. 84927/2001, voltage is applied between the upper electrode 904 and the lower electrode 903 of the electron source element 902 in a pixel, in which a signal is input into a scanning line to make the pixel TFT 901 ON, only for a period of time, during which a signal is input into a signal line, whereby electrons are emitted. Electrons are input into a fluorescent body only for a period of time, during which electrons are emitted, to cause a pixel emitting light.
For example, in the case where signals are input one pixel by one pixel from signal lines (dot sequential drive), a period of time, during which one pixel emits light, becomes equal to or less than 1/L of one frame period where the number of pixels possessed by a display device is L. Also, in the case where signals are input into all pixels in one column at the same time, that is, signals are input into pixels in one column at the same time from source signal lines S1 to Sx (line sequential drive), a period of time, during which one pixel emits light, becomes equal to or less than 1/y of one frame period assuming that a display possesses pixels of y columns.
Here, in the case where a display device such as large-sized displays, highly fine displays, or the like has a large number of pixels, a period of time, during which one pixel continues to emit light, becomes short in a display, in which pixels are constructed in the above manner. Therefore, when it is tried to represent an adequate luminance during one frame period, it becomes necessary to apply a high voltage between an upper electrode and a lower electrode of an electron source element in a short period of time. Therefore, a drive circuit is increased in drive voltage and load on elements, which constitute the drive circuit, becomes large. Therefore, there is caused a problem that a display device is degraded in reliability.
Also, in order to input analog signals into signal lines S1 to Sx, a plurality of signal voltages must be set to meet respective graduations. Therefore, there is caused a problem that such construction is not suited to multi-graduations.